A switched reluctance (SR) motor is a doubly salient variable reluctance motor. A coil, wound around each stator pole, is connected with at least one other coil on another stator pole, to form a phase winding. The reluctance of the flux path between a pair of stator poles varies as a respective pair of rotor poles rotates into and out of alignment with them. Because inductance is inversely proportional to reluctance, the inductance of a phase winding is at a maximum when the rotor is in the aligned position and at a minimum when the rotor is in the unaligned position.
Positive torque (i.e., "motoring torque") is produced if current flows in a phase winding as the inductance of that phase winding is increasing. A negative torque (i.e., "generating torque") contribution is avoided if the current is reduced to zero before the inductance begins to decrease again. Therefore, in a motoring operation, each phase is usually excited when its inductance is increasing, and is unexcited when its inductance is decreasing. In the case of a generating operation, the opposite is true. The rotor speed can be varied by changing the frequency of the phase current pulses while maintaining synchronism with the rotor position.
It is usually necessary to determine the angular position of the rotor relative to the stator, so that appropriate phase commutation (motor control operations) can be taken to properly and efficiently operate the motor. Some prior art systems for determining rotor position have relied upon the use of position sensing devices, which add substantial manufacturing costs to the motor. For instance, a hall effect sensor, a resolver or an encoder is located on the shaft of the motor. It is further noted that prior art position sensing devices may not be suitable for use in certain applications due to environmental considerations. For example, in an automotive application, the position sensing device may not function properly due the effects of dust, dirt and grime on the position sensing device. Alternatively, self inductance of a phase winding can be used to infer rotor position. Some prior art systems for sensorless rotor position detection have required the injection of currents into the phase winding. The rise time of the current in the phase winding is used to determine rotor position. This technique is disruptive of motor operations, and does not yield accurate position data. Accordingly, there is a need for a system for determining rotor position, which is inexpensive to manufacture, suitable for use in a wide variety of applications, including those in harsh environments, does not disrupt motor operations, and provides accurate position information.